Optical information recording medium

ABSTRACT

An optical information recording medium of this invention includes a substrate, a light incident surface, a first reflecting layer formed between the substrate and the light incident surface, a second reflecting layer formed between the first reflecting layer and the light incident surface and stacked on the first reflecting layer, the second reflecting layer being made of the same material as that of the first reflecting layer, and a phase change optical recording layer formed between the second reflecting layer and the light incident surface, the phase change optical recording layer transiting between a crystal state and an amorphous state when irradiated with a light beam.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2001-359607, filed Nov. 26,2001, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical information recording mediumcapable of recording and reproducing large-capacity information using alight beam.

2. Description of the Related Art

Recently, in the field of information recording, research into opticalinformation recording media and optical information recording methodsare progressing at various laboratories. Optical information recordingmedia can record/reproduce information in a noncontact state. Opticalinformation recording media are classified into read-only-type,write-once, read-many-type, and rewritable media and can cope withvarious memory forms. Such optical information recording media caninexpensively store large-capacity files and are expected to be widelyused as industrial and consumer devices.

CDs, LDs, and DVDs (Digital Versatile Discs) corresponding to aread-only memory form have already been widely proliferated. Theseoptical disks have a transparent substrate on which a three-dimensionalpattern such as pits and grooves that indicate an information signal isformed. A reflecting film formed from a metal thin film of, e.g.,aluminum is formed on the transparent substrate. A protective film forprotecting the reflecting film from oxidation is formed on thereflecting film. A light beam incident on the optical disk is reflectedby the reflecting film. The three-dimensional pattern such as pits andgrooves that indicate an information signal is reflected on reflectedlight reflected by the reflecting film. Hence, when a change inreflected light is detected, the information signal can be reproduced.

Phase change optical disks corresponding to a rewritable memory form arealready forming a market of PDs, DVD-RAMs, and DVD-RWs. The diskstructure will be described below. A transparent dielectric film isformed on a transparent substrate. A phase change recording layeressentially consisting of Ge, Sb, Te, In, or Ag is formed on thetransparent dielectric film. Another transparent dielectric film isformed on the phase change recording layer. A reflecting film made of,e.g., aluminum is formed on the transparent dielectric film. Inaddition, a protective film made of, e.g., a UV curing resin is formedon the reflecting film. Upon receiving a light beam from a semiconductorlaser, the phase change recording layer on the transparent substratereversibly transits between an amorphous state and a crystal state. Inan information reproduction mode, an information signal is reproduced bydetecting a change in reflected light from a recording portion of thephase change recording layer. In an information recording mode, arecording portion of the phase change recording layer is irradiated witha short-pulse light beam having a relatively high power to heat therecording portion to a temperature equal to or more than the meltingpoint. Then, the recording portion is quickly cooled to form anamorphous recording mark at the recording portion. In an informationerase mode, the recording portion of the phase change recording layer isirradiated with a long-pulse light beam having a lower power than in therecording mode to hold the recording portion at a temperature betweenthe crystallization temperature (inclusive) and the melting point(exclusive) or cool the recording portion from a temperature equal to ormore than the melting point, thereby crystallizing the recordingportion. As described above, in the phase change optical recording,information is recorded using a change in reflectance between theamorphous state and the crystal state. For this reason, an apparatus canhave an optical system with a simple structure. Phase change opticalrecording requires no magnetic field, unlike magnetooptical recording.Additionally, in phase change optical recording, an overwrite by lightintensity modulation is easy, and the data transfer rate is high.Furthermore, phase change optical recording has good compatibility witha read-only disk represented by a DVD-ROM and CD-ROM.

As a method of increasing the capacity of such an optical disk, the NA(Numerical Aperture) of the objective lens of an optical pickup isincreased to reduce the spot diameter of reproduction light, therebyattaining a high recording density. In a shift from, e.g., a CD to aDVD, the substrate thickness is decreased from 1.2 mm to 0.6 mm to copewith an optical system with a high NA. To increase the NA, thetransparent substrate through which reproduction light passes must bemade thinner. This is because when the NA is increased, the allowableamount of aberration generated by the angle of shift of the disk surfacefrom a plane perpendicular to the optical axis of the optical pickup,i.e., the tilt angle becomes small. For this reason, as the NAincreases, the transparent substrate must be made thin, and thesubstrate thickness distribution in the disk must fall within apredetermined range.

For a recording/reproduction optical disk such as a DVD, a light beambecomes incident from the substrate side. That is, a light beamirradiation surface in the reflecting layer is formed on the protectivelayer. An interface is formed between the reflecting layer and theprotective layer. Since the surface of the protective layer is reflectedon the light beam irradiation surface in the reflecting layer, anequilibrium is maintained. On the other hand, in a high-NA-compatibleoptical disk which is applied to an apparatus having an optical pickupwith a high-NA lens, layers are formed in an order reverse to that ofthe above-described conventional optical disk to ensure a tilt margin.In such a high-NA-compatible optical disk, a reflecting layer, secondprotective layer, phase change recording layer, and first protectivelayer are formed in this order. For this reason, the surface state ofthe reflecting layer is reflected on the second protective layer formedon the reflecting layer, the recording layer formed on the secondprotective layer, and the first protective layer formed on the recordinglayer. Generally, crystal grains on the surface of the reflecting layerformed from an AL alloy tend to have a large side due to columnar growthunique to a metal thin film. The surface roughness of the reflectinglayer also roughens the surface of the recording layer through thesecond protective layer. A mark recorded on the high-NA-compatibleoptical disk which aims at increasing density by increasing the NA isfiner than a mark recorded on the conventional optical disk. That is,the above-described surface roughness of the recording layer greatlyinfluences the recording/reproduction characteristic of thehigh-NA-compatible optical disk. More specifically, the surfaceroughness of the recording layer produces noise in the reproduction modeor causes strain at a mark edge in forming a recording mark. Hence, in ahigh-NA-compatible optical disk, such surface roughness(three-dimensional pattern) of the reflecting layer is preferablysuppressed.

In the conventional optical disk in which a light beam is incident fromthe substrate surface side, a reflecting layer is divisionally formedfor efficient mass production. With the divisional reflecting layerformation, the columnar growth of the reflecting layer is slightlysuppressed. However, the surface roughness of the reflecting layergenerates noise in the reproduction mode or fluctuates a mark edge at a1/10 wavelength or more. For this reason, even the above-describedcolumnar growth suppression by divisional film formation does notsuffice in obtaining a satisfactory recording/reproductioncharacteristic. In the conventional optical disk, an Al-based materialis used for the reflecting layer. However, this material readily formslarge-size crystal grains and is therefore unsuitable for ahigh-NA-compatible optical disk in which layers are formed in a reverseorder. As a method of reducing the surface roughness of the recordinglayer of a high-NA-compatible optical disk, a method of inserting ametal undercoat between the reflecting layer and the substrate isproposed in Jpn. Pat. Appln. KOKAI Publication No. 11-327890. However,this method has another problem that the disk manufacturing costincreases because an additional layer is formed.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide an opticalinformation recording medium having an excellent recordingcharacteristic.

According to an embodiment of the present invention, there is providedan optical information recording medium comprising a substrate, a lightincident surface, a first reflecting layer formed between the substrateand the light incident surface, a second reflecting layer formed betweenthe first reflecting layer and the light incident surface and stacked onthe first reflecting layer, the second reflecting layer being made ofthe same material as that of the first reflecting layer, and a phasechange optical recording layer formed between the second reflectinglayer and the light incident surface, the phase change optical recordinglayer transiting between a crystal state and an amorphous state whenirradiated with a light beam.

Additional objects and advantages of the present invention will be setforth in the description which follows, and in part will be obvious fromthe description, or may be learned by practice of the present invention.The objects and advantages of the invention may be realized and obtainedby means of the instrumentalities and combinations particularly pointedout hereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the presentinvention and, together with the general description given above and thedetailed description of the embodiments given below, serve to explainthe principles of the present invention.

FIG. 1 is a sectional view showing the layer structure of a disk A as anoptical information recording medium according to the embodiment of thepresent invention;

FIG. 2 is a sectional view showing the layer structure of a disk B;

FIG. 3 is a view showing the AFM image of the recording layer surface ofa sample a2;

FIG. 4 is a view showing the AFM image of the recording layer surface ofa sample b2;

FIG. 5 is a graph showing changes in CNR of closest patterns 3Tdepending on the repetitive numbers of times of recording of the disks Aand B;

FIG. 6 is a graph showing changes in jitter depending on the repetitivenumbers of times of recording of the disks A and B;

FIG. 7 is a graph showing a change in jitter depending on the repetitivenumber of times of recording of a disk obtained by replacing the AlTireflecting layers of the disk A with AgPdCu reflecting layers;

FIG. 8 is a graph showing changes in jitter in a disk obtained byreplacing the AlTi reflecting layers of the disk A with an AlMoreflecting layer and, more specifically, the relationship between Ra/t(t is the recording layer thickness) and the jitter in the first-timerecording and the relationship between Ra/t and the jitter afterrecording was performed 10,000 times;

FIG. 9 is a sectional view showing the layer structure of a disk havingthree protective layers; and

FIG. 10 is a sectional view showing the layer structure of a disk havingtwo recording layers.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the present invention will be described below.

In an optical information recording medium according to an embodiment ofthe present invention, a light incident surface, transparent coversheet, protective layer, phase change recording layer (to be referred toas a recording layer hereinafter), reflecting layer, and substrate areformed sequentially from one surface. That is, the reflecting layer isformed on the substrate. A light beam becomes incident from the oppositeside of the reflecting layer with respect to the substrate. As the mostimportant point, the reflecting layer is formed from first and secondreflecting layers. The first and second reflecting layers are made ofthe same material.

For the above-described recording layer, a material which transitsbetween a crystal state and an amorphous state upon receiving a lightbeam and exhibits different optical characteristics between the twostates is used. An example of this material is a ternary material suchas Ge—Sb—Te or In—Sb—Te. Even when at least one of Co, Pt, Pd, Au, Ag,Ir, Nb, Ta, V, W, Ti, Cr, Zr, Bi, Sn, and the like is added to thematerial in a very small amount, the recording layer can obtain asatisfactory characteristic. To realize a satisfactory recording/erasecharacteristic, the thickness of the recording layer is preferably 10 to20 nm. An average surface roughness Ra is preferably 1/50 to 1/10 of therecording layer thickness. When the surface roughness Ra is 1/50, thesurface energy becomes small, and the adhesion between the recordinglayer and a layer stacked on it decreases. Since the decrease inadhesion deteriorates the repetitive recording characteristic andshortens the storage life, a surface roughness Ra of 1/50 or less isunsuitable. When the surface roughness Ra is 1/10 or more, a change inlight absorbance along with a local change in film thickness of therecording layer cannot be neglected. This is undesirable because itcauses strain in the recording mark edge and lowers the jitter. With theabove structure, the recording layer can have a satisfactorycharacteristic. In addition, the reflectance when the recording layer isin the crystal state becomes lower than that in the amorphous state.

The protective layer in the optical information recording mediumaccording to the embodiment of the present invention mechanically andchemically protects the phase change recording layer. It also serves asan interference film for adjusting the optical characteristic of theoptical information recording medium. As the protective layer, atransparent dielectric film having a refractive index of 2.0 to 3.5 andan extinction coefficient of 0 to 0.2 is preferably used. A protectivelayer material preferably contains at least one of, e.g., Zn—S, Si—O,Si—N, Ti—O. Ge—N, Ta—O, Al—N, Cr—O, and SiC. Especially, a compositematerial containing Si—O and Zn—S is preferable. These protective layermaterials can also have an effect for promoting crystallization of therecording layer. Generally, since the stoichiometric ratio of a thinfilm material largely changes depending on the process condition, theratio of component elements is not specified.

The reflecting layer in the optical information recording mediumaccording to the embodiment of the present invention is indispensable toreflect an irradiation light beam and efficiently use the opticalenergy. The reflecting layer also has an effect of a heat dissipationlayer in controlling the heating/cooling process unique to the phasechange recording medium.

The reflecting layer in the optical information recording mediumaccording to the embodiment of the present invention is formed fromfirst and second reflecting layers made of the same material to make thesurface roughness as small as possible. The first reflecting layer isformed on the substrate. After a predetermined time, the secondreflecting layer made of the same material as that of the firstreflecting layer is formed on the first reflecting layer. Since thinfilm formation is temporarily stopped between the first and secondreflecting layers to stop crystal growth, growth of any large-sizegrains can be suppressed. At this time, a time of several secondssuffices between formation of the first reflecting layer and formationof the second reflecting layer. The time is preferably 5 seconds ormore. In dividing the reflecting layer, when the number of divisions ischanged in accordance with the material of the reflecting layer, thesurface roughness can be controlled in more detail. For a material suchas an Al-based material which readily causes large-size grain growth,the surface roughness reduction effect can be increased by dividing thereflecting layer into three or four layers. On the other hand, for amaterial such as Ag-based material with relatively small crystal grains,a sufficient effect can be obtained even by two layers. The surfaceroughnesses of the protective layer and recording layer formed on thereflecting layer having such a multi-layered structure are essentiallysmall. Hence, the recording/reproduction characteristic represented by ajitter characteristic considerably improves. As described above, only bydivisionally forming the reflecting layers in a plurality of steps (onlyby interrupting the reflecting layer formation process), the roughnessesof the surfaces of the reflecting layer and recording layer can besuppressed. That is, the recording/reproduction characteristic can beimproved without increasing the manufacturing cost of the opticalinformation recording medium. The reflecting layer material ispreferably an alloy mainly containing Ag, Al, or Au and more preferablyan Ag alloy that contains an additive element in 5 at % or less and hasan excellent resistance against an environment. The reflecting layer inthe optical information recording medium of the present inventionpreferably has a total thickness of 30 to 200 nm.

The optical information recording medium according to the embodiment ofthe present invention can easily be discriminated by observing itssectional shape with an electron microscope or the like. For example,when an optical information recording medium using an Al-basedreflecting layer according to the present invention is observed with atransmission electron microscope, the first and Al-based reflectinglayers are separated from each other.

The respective layers according to the embodiment of the opticalinformation recording medium of the present invention can be formed by ageneral physical deposition method. The layers can be formed by any filmformation method such as RF/DC sputtering, electron beam deposition,resistance heating deposition, or molecular beam epitaxy (MBE). In sucha thin film forming method represented by RF sputtering, the filmcharacteristic changes according to the process condition in filmformation. For example, a reflecting layer with a small surfaceroughness is preferably formed at a high growth rate. According to theoptical information recording medium of the present invention, thesurface roughnesses of the reflecting layer and recording layer can bereduced without forming any additional layer. Hence, the diskperformance can be expected to improve.

Example 1 of the optical information recording medium of the presentinvention will be described below.

The interior of a film forming apparatus was exhausted to 5×10⁻⁴ (Pa) orless. A DC power of 1 kW was applied to an AlTi target to form a 50-nmthick first reflecting layer on a 1.1-mm thick PC substrate. Ten secondsafter that, film formation was resumed to form a 50-nm second reflectinglayer. That is, AlTi reflecting layers having a total thickness of 100nm were formed. After that, an information layer was formed by RFmagnetron sputtering. That is, an RF power of 1 kW was applied to aZnS(80)-SiO₂(20) target to form a 15-nm protective layer. Subsequently,an RF power of 500 W was applied to a Ge target in a gas mixtureatmosphere containing argon and nitrogen to form a 5-nm thick GeN layer.Next, an RF power of 250 W was applied to a Ge₄₀Sb₈Te₅₂ target to form a15-nm thick phase change recording layer. Subsequently, an RF power of500 W was applied to a Ge target in a gas mixture atmosphere containingargon and nitrogen to form a 5-nm thick GeN layer. Next, an RF power of1 kW was applied to a ZnS(80)-SiO₂(20) target to form a 50-nm thickprotective layer. After that, the information-layer-side surface of thesubstrate extracted from the film forming apparatus was coated with a UVcuring resin. Then, a 0.1-nm thick polycarbonate (PC) cover layer wasbonded. The resultant structure was rotated by a spinner to decrease theresin thickness to several μm and irradiated with UV light to cure theresin layer. In this way, a disk A as the optical information recordingmedium of the present invention was completed. A disk B was prepared bycontinuously forming AlTi reflecting layers having a total thickness of100 nm and forming the remaining films under the same conditions andcompared with the disk A.

FIG. 1 is a sectional view showing the layer structure of the disk A asthe optical information recording medium of the present invention. Asshown in FIG. 1, a first AlTi reflecting layer 12 a, second AlTireflecting layer 12 b, ZnS(80)-SiO₂(20) protective layer 13, GeN layer14, Ge₄₀Sb₈Te₅₂ recording layer 15, Ge layer 16, ZnS(80)-SiO₂(20)protective layer 17, UV curing resin layer 18, and a 0.1-mm thick PCsheet 19 were sequentially stacked on a 1.1-mm thick PC substrate 11 ofthe disk A. Since the second reflecting layer 12 b was formed after theelapse of a predetermined time from formation of the first reflectinglayer 12 a, a boundary 12 c was present between the first reflectinglayer 12 a and the second reflecting layer 12 b. This disk A isirradiated with a light beam from the PC sheet 19 side. That is, theupper surface of the PC sheet 19 serves as a light incident surface 10.

FIG. 2 is a sectional view showing the layer structure of the disk B. Asshown in FIG. 2, an AlTi reflecting layer 22, ZnS(80)-SiO₂(20)protective layer 23, GeN layer 24, Ge₄₀Sb₈TeS₂ recording layer 25, GeNlayer 26, ZnS(80)-SiO₂(20) protective layer 27, UV curing resin layer28, and 0.1-mm thick PC sheet 29 were sequentially stacked on a 1.1-mmthick PC substrate 21 of the disk B. The reflecting layer 22 in the diskB had no boundary.

AlTi single layer samples a1 and b1 formed on Si substrates inaccordance with the same procedure as that for the disks A and B andsamples a2 and b2 for which a structure from an AlTi reflecting layer toa Ge₄₀Sb₈Te₅₂ recording layer were formed were prepared.

The AlTi single layer samples a1 and b1 were observed with atransmission electron microscope (TEM). The crystal grain distributionin a plane was observed. The average grain diameter was 34.2 nm for thesample a1 and 51.6 nm for the sample b1. The sections of the sampleswere observed with the TEM. For the sample a1, a boundary formed byinterrupted film formation was clearly observed. This revealed thatcrystal growth was suppressed. For the sample b1, crystal growth for thesubstrate reached the surface layer, and a large-size crystal wasformed.

The recording layer surfaces of the samples a2 and b2 were observed withan atomic force microscope (AFM). FIG. 3 is a view showing the AFM imageof the recording layer surface of the sample a2. FIG. 4 is a viewshowing the AFM image of the recording layer surface of the sample b2.The surface of the sample a2 was smooth relative to that of the sampleb2. The average surface roughness Ra was 2.7 nm for the sample b2 but0.8 nm for the sample a2, which exhibited a very satisfactorysmoothness. The surface roughness Ra of the sample b2 was 1/10 or moreof the recording layer thickness, 15 nm. However, the surface roughnessRa of the sample a2 was 1/10 or less of the recording layer thickness,15 nm. The influence of the difference in medium structure on therecording/reproduction characteristic was examined. Therecording/reproduction characteristics of the disks were evaluated underthe following conditions.

Laser output 0.1 to 6.0 mW Light source wavelength 405 nm Diskrotational speed 6.0 m/s Objective lens NA 0.85 Track pitch 0.30 μmShortest bit length 0.12 μm Recording pattern 3T, 11T Modulation scheme(8, 16) RLL

FIG. 5 is a graph showing changes in CNR of closest patterns 3Tdepending on the repetitive numbers of times of recording of the disks Aand B. The CNR of the disk A as the optical information recording mediumof the present invention maintained a high level of 52 dB or more for10,000 times. To the contrary, the CNR of the disk B remained 49 dB orless. The CNR of the disk A was high mainly because of low noise. Thisreflected that the AlTi reflecting layer had fine crystal grains, i.e.,the surface roughness of the recording layer was small.

FIG. 6 is a graph showing changes in jitter depending on the repetitivenumbers of times of recording of the disks A and B. The differencebetween the disks can be seen from the jitter characteristic shown inFIG. 6. The disk A has a satisfactory jitter value of about 8% while thejitter of the disk B exceeds 11%.

Comparison between the disks A and B proved that the disk A as theoptical information recording medium of the present invention had asmall three-dimensional pattern on the recording layer and exhibited anexcellent jitter characteristic.

Example 2 of the optical information recording medium of the presentinvention will be described next.

An optical disk was manufactured in accordance with the same procedureand layer structure as in the disk A of Example 1 except that thereflecting layers (first AlTi reflecting layer 12 a and second AlTireflecting layer 12 b) were changed to two 25-nm thick AgPdCu layers,i.e., layers having a total thickness of 50 nm. FIG. 7 is a graphshowing a change in jitter depending on the repetitive number of timesof recording of the disk obtained by replacing the AlTi reflectinglayers of the disk A with the AgPdCu reflecting layers. Due to theeffect of the AgPdCu reflecting layers having a smaller grain diameterthan the AlTi reflecting layers, the jitter characteristic became muchbetter than disk A. A jitter of 8% or less was obtained in repetitiverecording of 10,000 times.

Example 3 of the optical information recording medium of the presentinvention will be described next.

An optical disk was manufactured in accordance with the same procedureand layer structure as in the disk A of Example 1 except that thereflecting layers (first AlTi reflecting layer 12 a and second AlTireflecting layer 12 b) were changed to an AlMo reflecting layer. Asurface roughness Ra of the recording layer was changed by appropriatelychanging the number of divisions of the AlMo reflecting layer. Thesurface roughness Ra was obtained through AFM observation. FIG. 8 is agraph showing changes in jitter in the disk obtained by replacing theAlTi reflecting layers of the disk A with the AlMo reflecting layer and,more specifically, the relationship between Ra/t (t is the recordinglayer thickness) and the jitter in the first-time recording and therelationship between Ra/t and the jitter after recording was performed10,000 times. The jitter in the first-time recording exhibited asatisfactory characteristic of about 8% when Ra/t≦0.1. To the contrary,the jitter after recording was executed 10,000 times exhibited anincrease not only when Ra/t>0.1 but also when Ra/t<0.02. That is, adegradation in disk performance was observed. As can be seen from theabove results, the ratio of the surface roughness Ra to the recordinglayer thickness t was appropriately 0.02 to 0.1. As described above, theeffect of the present invention was confirmed even in a disk having adifferent reflectance polarity.

Example 4 of the optical information recording medium of the presentinvention will be described next.

As shown in FIG. 9, a disk was manufactured in which AlMo (50 nm*3)(i.e., AlMo (50 nm) reflecting layer 32 a, AlMo (50 nm) reflecting layer32 b, and AlMo (50 nm) reflecting layer 32 c), ZnS—SiO₂ (15 nm)protective layer 33, GeN (5 nm) layer 34, Ge₃₀Sb₁₆Te₅₄ (13 nm) recordinglayer 35, GeN (5 nm) layer 36, ZnS—SiO₂ (20 nm) protective layer 37,SiO₂ (60 nm) protective layer 38, ZnS—SiO₂ (30 nm) protective layer 39,UV curing resin layer 40, and PC sheet 41 were sequentially stacked on a1.1-mm thick PC substrate 31. This disk is irradiated with a light beamfrom the PC sheet 41 side. That is, the upper surface of the PC sheet 41serves as a light incident surface 30. The AlMo reflecting layer wasdivided into three layers (32 a, 32 b, and 32 c) at a wait time of,e.g., 6 seconds. AlMo reflecting layers having a total thickness of 150nm were formed. Section TEM observation showed that the grain growth wasinterrupted, and two boundaries 32 d and 32 e were present between thedivided portions schematically shown in FIG. 9. The jittercharacteristic was evaluated, as in the above examples. The jitterexhibited a satisfactory characteristic that fell within the range of7.5% to 8.5% for repetitive recording of 10,000 times or more. InExample 4, the reflectance before recording was about 5%, i.e., lowerthan that after recording. When the reflectance is low, focusing ortracking readily becomes difficult. Especially in a high-NA-compatiblemedium, this tendency gets stronger by the three-dimensional pattern onthe reflecting layer. When the reflecting layer was divided, as inExample 4, stable tracking was obtained even in a disk having such areflectance polarity.

Example 5 of the optical information recording medium of the presentinvention will be described next.

As shown in FIG. 10, AlTi (50 nm*2) (i.e., AlTi (50 nm) reflecting layer52 a and AlTi (50 nm) reflecting layer 52 b), ZnS—SiO₂ (30 nm)protective layer 53, GeN (3 nm) layer 54, second Ge₂Sb₂Te₅ (13 nm)recording layer 55, GeN (3 nm) layer 56, and ZnS—SiO₂ (90 nm) protectivelayer 57 were sequentially formed on a 1.1-mm thick PC substrate 51 bysputtering. The AlTi reflecting layer was divided into two layers (52 aand 52 b) at a wait time of, e.g., 8 sec. AlTi reflecting layers havinga total thickness of 100 nm were formed. Parallelly, a ZnS—SiO₂ (30 nm)protective layer 64, GeN (2 nm) layer 63, first Ge₄₀Sb₈Te₅₂ (6.5 nm)recording layer 62, GeN (2 nm) layer 61, ZnS—SiO₂ (25 nm) protectivelayer 60, and AgPdCu (10 nm) layer 59 were sequentially formed bysputtering on a 0.1-mm thick PC sheet 65 having the same track shape asthat of the PC substrate 51. The film surfaces of the two media werebonded via a 30-μm thick UV curing resin layer 58. In such an opticaldisk having two recording layers, a light beam incident from a lightincident surface 50 is independently focused on the first and secondrecording layers to allow recording/reproduction on the respectivelayers. Hence, the capacity of the disk can be increased. Section TEMobservation showed that the grain growth was interrupted, and a boundary52 c was present between the AlTi reflecting layers schematically shownin FIG. 10. The jitter characteristic was evaluated. The jitterexhibited a satisfactory characteristic that fell within the range of8.5% to 9.0% in the first recording layer and the range of 7.5% to 8.0%in the second recording layer for repetitive recording of 10,000 timesor more.

Especially in the second recording layer, a smaller jitter was obtained,and the effect was confirmed.

As has been described above in detail, an excellent jittercharacteristic can be obtained using the optical information recordingmedium according to the present invention.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. An optical information recording medium comprising: a substrate; alight incident surface; a first reflecting layer formed between saidsubstrate and said light incident surface; a second reflecting layerformed between said first reflecting layer and said light incidentsurface and stacked on said first reflecting layer, said secondreflecting layer being made of the same material as that of said firstreflecting layer; and a phase change optical recording layer formedbetween said second reflecting layer and said light incident surface,said phase change optical recording layer transiting between a crystalstate and an amorphous state when irradiated with a light beam, whereinan average surface roughness Ra of said recording layer is 1/50 to 1/10(both inclusive) of a thickness of said recording layer.
 2. An opticalinformation recording medium comprising: a substrate; a light incidentsurface; a first reflecting layer formed between said substrate and saidlight incident surface; a second reflecting layer formed between saidfirst reflecting layer and said light incident surface and stacked onsaid first reflecting layer, said second reflecting layer being made ofthe same material as that of said first reflecting layer; and a phasechange optical recording layer formed between said second reftectinglayer and said light incident surface, said phase change opticalrecording layer transiting between a crystal state and an amorphousstate when irradiated with a light beam, wherein said first and secondreflecting layers contain an Ag alloy.
 3. An optical informationrecording medium comprising: a subsrtate; a light incident surface; afirst reflecting layer formed between said substrate and said lightincident surface; a second reflecting layer formed between said firstreflecting layer and said light incident surface and stacked on saidfirst reflecting layer, said second reflecting layer being made of thesame material as that of said first reflecting layer; and a phase changeoptical recording layer formed between said second reflecting layer andsaid light incident surface, said phase change optical recording layertransiting between a crystal state and an amorphous state whenirradiated with a light beam, wherein a reflectance when said recordinglayer is in the crystal state is lower than that when said recordinglayer is in the amorphous state.
 4. An optical information recordingmedium comprising: a substrate; a light incident surface; a firstreflecting layer formed between said substrate and said light incidentsurface; a second reflecting layer formed between said first reflectinglayer and said light incident surface and stacked on said firstreflecting layer, said second reflecting layer being made of the samematerial as that of said first reflecting layer; a phase change opticalrecording layer formed between said second reflecting layer and saidlight incident surface, said phase change optical recording layertransisting between a crystal state and an amorphous state whenirradiated with a light beam; and a protective layer formed between saidphase change recording layer and said light incident surface and formedfrom three layers.
 5. An optical information recording mediumcomprising: a substrate; a light incident surface; a first reflectinglayer formed between said substrate and said light incident surface; asecond reflecting layer formed between said first reflecting layer andsaid light incident surface and stacked on said first reflecting layer,said second reflecting layer being made of the same material as that ofsaid first reflecting layer; and a phase change optical recording layerformed between said second reflecting layer and said light incidentsurface, said phase change optical recording layer transiting between acrystal state and an amorphous state when irradiated with a light beam,wherein said phase change recording layer includes a plurality ofrecording layers capable of independently recording/reproduction.
 6. Anoptical information recording medium comprising: a substrate; a lightincident surface; a first reflecting layer formed between said substrateand said light incident surface; a second reflecting layer formedbetween said first reflecting layer and said light incident surface andstacked on said first reflecting layer, said second reflecting layerbeing made of the same material as that of said first reflecting layer;and a phase change optical recording layer formed between said secondreflecting layer and said light incident surface, said phase changeoptical recording layer transiting between a crystal state and anamorphous state when irradiated with a light beam, wherein saidplurality of recording layers are isolated from each other byintermediate layers.